The document discusses troubleshooting techniques for computer systems. It covers: - The process of troubleshooting involves observing symptoms, analyzing them, diagnosing faults, and rectifying issues. - Symptoms can include program errors, hardware failures, and stability problems. Faults can be permanent or intermittent. - Troubleshooting requires understanding the computer's architecture, organization, engineering layout, and potential environmental factors. - A variety of tools are used including logic probes, pulsers, oscilloscopes, and diagnostic programs. The goal is to methodically determine if the issue is hardware or software related. - Systematic troubleshooting is more effective than ad hoc methods and requires carefully observing symptoms, analyzing relationships between symptoms, and

1. Trouble Shooting
Trouble shooting is the process of tracing and correcting faults in a mechanical or electronic system.
Trouble shooting in a computer is an intelligent and sophisticated process. Successful trouble shooting
depends on the following four factors:
- Problem solving approach
- Techniques adopted in trouble shooting
- Tools and test equipment used
- Diagnostics aids in PC
Computer Faults:
A fault in a computer system affects the normal operation of the computer, i.e, the computer
malfunctions. The symptoms of malfunction are almost unlimited and vary with the nature of the fault.
The types of symptoms we often come across are listed below:
- A program has given wrong results.
- A program is looping continuously
- The CPU has lost control
- There is a junk display on the CRT monitor
- The hard disk drive is not working
- Opening more than one window causes the system software to hang
- Computer intermittently cancels job with any of the following messages:
o Spurious Interrupt
o Memory parity error
o File not found
o Divide by Zero
o Overrun
- A program gives different results at different runs.
Nature of faults: solid and intermittent
Computer faults can be broadly classified into 2 types based on the frequency of occurrence of the
program.
- Permanent or Solid fault: When there is a permanent fault in a computer the computer
misbehaves consistently. Any number of times the program is run, the result or symptoms will
be same
- Intermittent fault: When there is an intermittent fault, the computer’s behavior is not
consistent. Sometimes it works properly and suddenly it malfunctions. After a certain period of
time, it recovers from the fault automatically and again starts functioning properly.
Types of Faults: (Hardware & Software):
A hardware which works now may fail even in the next second. But a software one proven always works.
Hence if there is a software fault in a computer, it is because of one of the following two reasons:
- The program is not proven. It is a new program which is not yet debugged thoroughly

2. - The program works only for certain types of data. This is due to the incomplete testing of the
software.
Software errors may appear to be hardware errors. One must clearly understand that a working
software can never malfunction.
A Hardware problem is classified as a fault in an actual component or in the configuration of that
component.
Hardware problems are of many types:
- Electronic Problems
- Mechanical Problems
- Environmental problems
- Media problems
Each of these types may cause both solid and intermittent problem. They may cause a similar
problem. In practice, it is easier to identify the cause of solid problems. An intermittent mechanical
problem may give symptoms that may mislead the engineer to conclude it as an electronic problem.
The symptoms caused by the media problems make one think that they are either hardware or
software problems. Hence an intermittent problem needs a very careful and systematic approach.
Diagnostics Programs and Tools: Computer manufacturers supply special diagnostic programs.
These programs contains test routines to verify the various functions of the computer hardware.
Running these programs help one to decide whether the hardware is faulty or not. In order to verify
the presence of intermittent problems, it is necessary to run the diagnostics multiple times. These
test programs very often help us to confirm whether a problem in a computer is related to hardware
or software. If diagnostic programs runs successfully without pointing out any problems. Then the
chance of a hardware fault is remote. But there are two exceptions to this:
- Intermittent problems may play a hide and seek game.
- Some problems may be too complex to be detected by simple diagnostic programs. These faults
may occur only when several subsystems work simultaneously.
Unless diagnostic programs simulate the above two situations, then faults may not be caught by
the diagnostics programs. (If these two exceptional cases are neglected, the service engineer
will be looping indefinitely in wrong areas). In this cases, the engineer has to look for alternative
ways of troubleshooting.
Rest out of Diagnostic Programs:
Methods of operation eg.
- Black box, which is testing of a mechanism without knowing how it works, and merely focusing
on the accuracy of output data based on a known input.
- White box, which uses knowledge of a mechanisms inner functions to direct testing.
- Background diagnostics, that perform testing of systems components during idle-time of a
system.
Rest out of nodal testers:

3. Logic Prob: it is handheld pen-like-test probe used for analyzing and troubleshooting the logical
states(Boolean 0 or 1) of a digital circuit. These are usually differently coloured LEDs on the probe’s
body:
- Red and green LEDs indicate high and low states respectively
- Amber LED indicates a pulse
Trouble Shooting Tools:
There are 2 types of trouble shooting tools:
- Nodal testers: Nodal testers are simple conventional test equipment used for probe in and
around the fault circuit. These testers include oscilloscope, logic probe, logic clip, logic pulser,
current tracer and comparator
o Logic Probe- logic probe is a handheld low cost test equipment with which nearly ninety
percent of the fault can be diagnosed, without an oscilloscope which is 10 to 20 times
costlier. It is used to find out the logic state of any node in a circuit including steady
states and pulses. The internal circuit in a logic probe is powerful to detect even very
narrow pulses.
o Logic Pulser: is a handheld tool used to inject pulses at the input of a gate under test. A
single pulse or a stream of pulses at different frequencies is issued as per the user
choice.
It is capable of delivering pulses of various computations, to any type of circuit we wish
to test. It is needed to properly test and locate a fault in digital circuits. It becomes very
useful when a tricky fault comes in. It becomes useful when circuits require to be tested
in sections. This means the input waveform is not present and we require to know if the
circuit will process the signal when it all goes together. It is capable of delivering pulses
to a circuit so that the result(s) can be detected, even though the circuit may not be
complete.
o Current Tracer: is a handheld tool which detects current flow in electronic circuits. It is
useful in locating shorted components, track shorts, solder bridges, Vcc to ground shorts
etc.
It detects current activity on Logic nodes. It can be used to identify current paths and
relative magnitudes. The electromagnetic field generated by a change in current is
detected by the inductive pick-up on the tip of the tracer. The tracer only responds to a
change in current, not DC current. A lamp indicator shows the relative magnitude of
current going through the node. The brighter the light gets, the more current in going
through the node.
o Oscilloscope: helps to resolve almost any problem in a PC.
It is a type of electronic test instrument that allows observation of constantly varying
signal voltages, usually as a two-dimensional plot of one or more signals as a function of
time. Non-electrical signals (such as sound or vibration) can be converted to voltages
and displayed. The observed waveform can be analyser for such properties as
amplitude, frequency, rise time, time interval, distortion etc.
It is useful is these levels of testing:

4.  Test logic level
 Measure the signal characteristics such as frequency, pulse width, rise and fall
time, ringing, noise etc.
 To measure the timing relationship between 2 or more signals
- System Testers: are sophisticated test equipment which are useful to quickly isolate the faulty
system. The system testers include emulators, functional testers, signature analyzers and logic
analyzers.
Fault Elimination Process:
Fault finding is a technique. Some faults can be easily traced out if one proceeds systematically,
starting from the symptoms. Some faults show complex symptoms with no clue at all. For such faults
there are no defined starting points. In such case, service engineers should try out an appropriate
elimination process.
The steps of fault elimination procedure are common to any computer. Some of these can be
omitted depending on the symptoms. Intuition is necessary for the progress of troubleshooting.
Dead System: A computer is said to be dead when an absolutely essential signal is absent. It is easy
to confirm whether a system is dead or alive. Following are some of the symptoms of a dead
computer:
- dc voltage is absent
- clk signal is absent
- ROM output is inactive
- Microprocessor is faulty or dead
- The ac fuse is blown
Spurious Problems: Sometimes the computer malfunctions randomly. Logical and systematic
trouble shooting may not yield any clue. In such cases the engineer should look for symptoms of
spurious problem. Some of the hidden symptoms of spurious problems are-
- dc voltage has excessive ripples
- dc voltage is noisy
- clk signal is not stable
- loose contacts of cables and connectors
- there is noise pick up on the next input or on other bus signals
Security Failures: In any computer, sometimes there are abnormal symptoms, due to
malfunctioning of special hardware meant for error detection.
Three types of problems are caused by such security logic:
- When there is no error, the error detection logic (Eg. ECC ch..) raises a false alarm.
- When there is an error, the security guard sleep without catching it. The error propagates and
creates confusion.

5. - The error detection hardware malfunctions in generating appropriate coding. For example, the
parity generator may generate a wrong parity bit. This goes unnoticed immediately when the
parity checker detects an error, the OS wrongly considers it a genuine error.
Heart Beats:
We have seen some of the symptoms of a dead PC earlier. These are the general symptoms
applicable to any computer. In addition, there are some more check points which are the heart
beats of PCs. By sensing these, the engineer can predict the problem type and determine how to
proceed with diagnosis. Two such effective check points are-
o Memory Refresh Request: Refresh in DRAM IC is done at regular interval. For this
purpose, refresh signal is generated once every 15 µsec. if this pulse is present it
indicates the following-
 Microprocessor is alive. Initial start-up is successful. Reset sequence had been
completed.
 Microprocessor firmware has successfully initialized the refresh counter
 The refresh logic is functioning properly
 The data bus, address bus and control bus are reasonable error free
o Timer 0- Ticks: The PC maintains the time of day. This time is periodically updated by
the cooperation between timer 0 and timer interrupt service routine. The timer 0(PIT)
raises Tick periodically and the system uses these interrupt signals to update the time.
The generation of the Tick can be monitored by probing IRQ0 signal from PIT.
Systematic Troubleshooting
Adhoc techniques of troubleshooting don’t always help an engineer. Systematic trouble shooting is a
logical approach. It is a scientific and analytical process. The systematic troubleshooting approach can be
divided into the following steps:
- Symptoms Observation
- Symptoms Analysis
- Fault Diagnosis
- Fault Rectification
Symptoms Observation:

6. The first step in troubleshooting a system or peripheral is observing all the symptoms caused by the
fault in the system. For some problems, especially for problems due to multiple faults, it will be
impossible to locate the exact fault or it will take a long time & thus the downtime will be increased. The
time spent on symptoms observation in turn will yield proportional return during fault locating. Clean
mind and concentration is essential while troubleshooting a problem. Certain complex problems can be
understood only if the engineer has an overall idea of the system functioning of various software
routines, and sequences performed by POST,BIOS and OS
Symptoms Analysis:
A careful analysis of the symptoms will give a clue to the fault location process. When there are multiple
symptoms the engineer should try to correlate the various symptoms to identify the problem. If any
relationship or clue is found, several steps can be skipped during fault location, thereby reducing the
downtime considerably. In cases where there is no interrelation between the symptoms of a problem,
the engineer should proceed with the fault location, without wasting time in attempting to establish the
relationship.
If multiple faults are present, then we may be minted by mixed symptoms. It is difficult to predict the
presence of multiple faults. Two probable clues are
- Symptoms are not consistent but keep changing
- Certain Symptoms vanish suddenly
Troubleshooting a system with multiple faults is a tough task. We may have to repeat the test multiple
times. Once multiple fault is suspected, one should be strictly systematic.
Fault Diagnosis:
It is a highly sophisticated scientific process. The most important point here is the multi-dimensional
view to be applied by the hardware engineer while troubleshooting problems. The main dimensions are-
- Architecture: Certain problems can be visualized properly only when you analyze the problem
and the behavior of the computer as a system consisting both of hardware and software.
Software may be either a system software or an application software. It is not necessary that we
should be a programmer. But we should know what the programs do, when they start and stop
some activities and when they interact with hardware.
- Organization: Different faults may create the same external symptom. The engineer must
distinguish the different faults by a deep analysis of the internal machine status. Every computer
has certain unique organizational aspects. Before trouble shooting a computer, the engineer
should understand the specific details of organization of the computer.
The computer with the same architecture need not have the same organization. The hardware
organization refers to functional and physical distribution of various subsystems in the
computer. It is decided by the computer designers based on the status of computer technology
during the design stage.
Hardware engineer should know the organizational aspects including detailed timing
relationships and signal sequences. Even if the engineer fortes all the signals in a computer
system, he won’t be able to locate the fault if he does not know how to correlate the various
signal conditions.

7. Engineering Layout: Certain problems are caused by mere physical layout faults:
Faulty cable, loose connection, open wire, track short, etc.
To catch these problems, one must thoroughly analyze the physical aspects.
The layout documents related to a computer are-
Circuit diagram
Board layout diagram
Cable signals list
Connecter signals list
Jumper settings
Environment: Certain problems are caused due to environmental conditions and are listed below:
High Temperature
DUST
Magnetic fields
Humidity
Fault Rectification: Once a problem is diagnosed thoroughly and the fault is traced, the rectification
involves skilled mechanical work rather than analytical work. This step may involve performing one or
more of the actions depending on the result of fault diagnosis.
- Replacing a defective IC or other components
- Replacing a defective PCB
- Cleaning the R/W heads
- Formatting a diskette or hard disk
- Adjustments: mechanical or electrical
- Controlling temperature or improving cooling.
Microprocessor:
The microprocessor is a boon for designers. At the same time it is a headache to service engineers and
technicians. Troubleshooting any microprocessor based equipment needs special skills because of the
sophisticated capabilities of the microprocessor. The microprocessor is a smart and intelligent
component. Unlike non-intelligent ICs, its outputs cannot be simply correlated to inputs. A
microprocessor behaves according to the instruction it has fetched.
A microprocessor’s internal activity can be guessed to some extent by carefully analyzing its output
signals but an overall picture can be gained only if we know the instruction being executed by the
microprocessor. One must know the program sequence in a microprocessor based equipment in order
to troubleshoot faults systematically and meaningfully.

8. In almost every microcomputer, there is a firmware paced in ROM. A service engineer must have a clear
idea about the functions and the sequence of execution of various firmware routines.